900 Overbore Advice

Hello all,

I am seeking a bit of advice from those who have done it before...

There is a LOT of information/mis-information out there about what will work and what will not...

I have a 1976 900 A4 (903cc). It is due for a real re-build. I figure, why go all that way and not improve what I have?

My question is this: What are the options for a streetable 900? I have 900 bottom and top end. I also have a 79' 1000 parts bike in my back yard of questionable history.

I am looking for the best size which doesn't require re-sleeving. I was under the impression that the difference between a stock 900 and 1000 was not the bore but the stroke as changed by connecting rod length. I have been looking at the 1015 kit (Stock 1000?), 1045, and 1075 to mate with my 900 bottom end.

Any advise is appreciated. I know this topic has been gone over before but my search results look like a "what oil do you use" debate

900 can be bored out to 70mm (1015) before resleeving and crankcase machining.

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Both the 903cc and 1015cc engines have the same stroke 66mm. The bore went from 66mm in the 903 to 70mm in the 1015. This is for the engines made before 1981.

I just installed a 1000 set of jugs and pistons. Check your other engine out and see if the jugs and pistons are good.

903 cylinder can be bored to 1015 cc max......1015 cylinder can be bored to 1075 cc max :dry: stroke is the same for 903 and 1015 :S

I say this Because over time ended up going the same thought process / route....

Started w/1015cc build, then wanted more umphka... :pinch: Should of just went 1075cc from the start for I have built this engine so many times now it's ridiculous... :whistle:

Take the 900 engine out and have the KZ1000 engine bored to 1075... This way you won't have to bore open the top case for the cylinder sleeves. All internals are for the most part a direct swap, crank, tranny, drive shafts, engine cases etc...

NOTE: W/1075 displacement, definitely replace timing chain, guides, rubber dampers etc....
Replace all engine seals & gaskets. Check the crank bearings as well. If not truly racing, crank doesn't need to be welded for a 1075 build. Yes better if you do and have the funds but not required for normal driving, occasional throttle twisting.

And you'll have a good KZ900 engine sitting there for later use, another build whatever...

You'll gain ~ 200 cc displacement!!! Other than the cylinder 1015 stamping, no one will know for the most part. Twist her up on another bike and they will be surprised how the old girl pulls away on them.

Thanks for the advice everyone. Glad to be cleared up on the 900/1000 stroke idea. Not sure where that came from.

OMR, the only reason I would rather not use that 1000 engine is because it is a shaft drive model and in really rough shape.

Could I use the cylinder jug from the 1000, go to 1075, and put that in my 900 bottom end with 900 head? I guess that is what I am starting to aim at...

And yes, the rebuild will include many new parts as you mentioned. My gear shifter splines are pretty haggard so I might as well go a little deeper and fix that as well.

yes you can use 1000 jug on 900 bottom end as well as head but........ be advised that you may have to relieve the cases slightly for the 1000 cylinders to set on i have done them W/O and some required it. at this time i would check crank and trans as well as clutch hub. i have a good crank if needed already done and ready to install.

You shouldn't have to touch the cases, I did the same thing and it all bolted together without a problem. 903 engine also has a lighter crankshaft than the 1015 motor. Add a set of vm28 carbs and you've got the best of the early stock components.

I've have seen some 900 upper case halves that do require relieving for the 1015 cylinder liners .....some 1015 cylinders will go right on , no problem :blink: ..... depends on the cases. and it's usually very minor relieving :pinch: if needed.. :huh:

turboking wrote: I've have seen some 900 upper case halves that do require relieving for the 1015 cylinder liners .....some 1015 cylinders will go right on , no problem :blink: ..... depends on the cases. and it's usually very minor relieving :pinch: if needed.. :huh:

This is correct some will work some need some minor work.

Cool, thanks for the advice. This is looking like my best option to get displacement with the engine and parts I have without having to re-sleeve. OMR - Did you send your jug to Pit Stop for boring and rough installation of the 1075 pistons as per your picture?